This is more of a mysql feature to protect against attacks. But a default limit of 10 for max_connect_errors can some times be a problem.
This situation could arise if there is a script under some domain on your server, which is consistently trying to connect to the mysql database using invalid login information.
We would suggest that you increase the max_connect_errors, in steps of 50 starting with 250, and observe if this problem decreases or is eliminated.
You will need to add the following in /etc/my.cnf under section [mysqld] set-variable=max_connect_errors=250
And then restart mysql using the following command:
service mysqld restart
Please note, we have not yet tested the impact of this option, if you face any problems, please remove it. Though there should be no issue setting this mysql variable (max_connect_errors) to a reasonably higher value than the default of 10.
You may also want to create a cron job to periodically run 'mysqladmin flush-hosts'.